Hi Rbacch,
When you import your mesh, depending on the UV layout for the lightmap you’ll likely need to increase your Lightmap Resolution for the mesh. By default it will use a value of 64, so you may need to adjust the value to something higher like 128 or 256 to get better results, then rebuild lighting and see if that helps.
If you’re still having issues, please post a screen shot of your Lightmap UV (UV Channel 1). You can do this by going to the Static Mesh Editor and hitting the UV button in the toolbar. Then in the drop down next to it, select UV Channel 1.
Thank you!
Tim